1. Field of the Invention
This invention relates to a hysteresis circuit, and more particularly, to a hysteresis circuit having a wide hysteresis range and a stable zero cross level.
2. Description of the Prior Art
In the past, circuits using direct current amplifiers and Schmidt trigger circuits have been widely used as hysteresis circuits. In a hysteresis circuit using a direct current amplifier, as shown in FIG. 1(a), an input voltage V.sub.in is supplied to the inversion output terminal of a direct current amplifier 1 whose output voltage is divided by resistors R1 and R2. The divided voltage is supplied to the non-inversion input terminal of the amplifier 1.
FIG. 1(b) shows the relationship between the input voltage and the output voltage of the circuit of FIG. 1(a). In this input and output voltage characteristic diagram, the threshold voltage V.sub.t is expressed by the following equation: ##EQU1## WHERE V.sub.out is the output voltage of the circuit and R1 and R2 are the values of the resistances used for voltage splitting.
From the above equation, it is clear that in order to change the hysteresis range, R1 or R2 or the output voltage V.sub.out may be changed. In this case, the method of changing the value of a resistance is generally adopted. However the threshold voltage V.sub.t is proportioned to ##EQU2## and, therefore, if R1 or R2 is changed linearly, the hysteresis range is changed non-linearly. Expressing this conversely, if it is desired to change the hysteresis range linearly, then R1 or R2 must be changed non-linearly.
Accordingly, if the change in the resistance value is to be made electronically, the electronic circuit for this purpose becomes rather complicated. This becomes a great disadvantage in cases where the circuit is of integrated construction.
On the other hand, a Schmidt trigger circuit is well known as a circuit having a hysteresis characteristic. FIG. 2 shows a typical Schmidt trigger circuit which includes a transistor Q1 and a transistor Q2. In this circuit, the emitters of the transistors Q1 and Q2 are connected together and to ground via a resistor R1, and the collector of the transistor Q1 is connected to the base of the transistor Q2 via a resistor R2. The base of the transistor Q2 is also connected to ground via a resistor R3. Each of the collectors of the two transistors is connected to a control voltage input terminal V.sub.cc via respective resistors R4 and R5. The input voltage is applied to the base of the transistor Q1. The output voltage is taken from the collector of the transistor Q2.
In this circuit, if the value of the voltage applied to the base of the transistor Q1 is below a threshold value voltage, then the transistor Q2 will be on, and, if the value of the voltage applied to the base of Q1 is above the threshold value voltage, then the transistor Q2 will be off. Thus, the arrangement is such that an output voltage is obtained only if the input voltage is above the threshold value. By making the loop gain of this circuit greater than 1, it is possible to give the circuit a hysteresis characteristic. However, it is well known that it is generally impossible to vary the loop gain of such a circuit over a wide range. Accordingly, this circuit is not suitable for employment in cases where positive use is made of its hysteresis characteristic.
The two known techniques described above both have the disadvantage that the zero cross level is unstable. That is to say, in the circuit shown in FIG. 1(a), the zero cross level is made unstable by the temperature characteristics of the transistor constituting the direct current amplifier. Likewise, in the circuit shown in FIG. 2, the threshold values of the transistors Q1 and Q2 have temperature characteristics and therefore the zero cross level becomes unstable.